In the field of watch making, a striking mechanism may be added to a watch movement in order to generate a sound or piece of music. The gong of the striking watch or the vibration plate of the musical watch are generally arranged inside the watch case. Thus, the vibrations of the gong or the vibration plate strips are transmitted to the various external parts of the watch. These external parts are, for example, the middle part, the bezel, the crystal and the back cover of the watch case. These large external parts start to radiate sound into the air under the effect of the transmitted vibrations. When a sound is produced either by a gong struck by a hammer, or by one or more vibrating strips of the vibration plate, these external parts are capable of radiating the produced sound into the air.
Usually, in a striking or musical watch of this type, acoustic efficiency, based on the complex vibro-acoustic transduction of the external parts, is low. In order to improve and increase the acoustic level perceived by the user of the striking or musical watch, the material, geometry and boundary conditions of the external parts must be taken into account. The configurations of these external parts are also dependent upon the aesthetic appearance of the watch and operating stresses, which may limit adaptation possibilities.
To further improve the vibro-acoustic efficiency of the striking mechanism, a membrane can be arranged inside the watch case. The membrane must be dimensioned so that all the notes generated by the vibration of one of more gongs, or of the vibration plate strips are efficiently radiated. It is therefore important that the frequency of these notes is close to the natural modes of the membrane in order to allow it to enter into resonance.
It is noted in this regard that a high modal density over a limited frequency bandwidth, for example between 500 Hz and 3.5 kHz. is hard to obtain using standard uniform membranes, since this characteristic is only compatible with uniform membranes of very low stiffness or of very high mass. The two characteristics are not advantageous, since reducing the frequency of the first resonant mode to around 1,000 Hz in this manner also reduces the frequency of the excited modes, whose acoustic performance is very limited, to below 4,000 Hz. The mechanical energy is therefore dissipated in vibration modes of the membrane having poor acoustic efficiency. The radiation efficiency, which is logically defined as the ratio between the radiated acoustic wave energy divided by the total energy transferred to the membrane, is therefore reduced over almost the entire frequency range of interest. It is therefore hard to obtain a resonance on every note generated by the striking mechanism, which constitutes a drawback of state of the art membranes.
EP Patent Application No. 2 461 219 A1, which discloses an acoustic radiating membrane for a musical or striking watch, may be cited in this regard. This acoustic membrane has a general dome shape with its peripheral edge sandwiched between part of the middle part and the back cover of the watch case. This membrane is designed with one or two asymmetrically shaped areas formed in the material of the membrane. The two areas excavated in the general thickness of the membrane are of different dimensions. These two areas form ellipses, which are offset from each other relative to the centre of the membrane and partly superposed. With these ellipses in the membrane, it is possible to have twice the number of modes of vibration for each ellipse in comparison with a circular shape. However, this does make it possible to increase the bandwidth of vibration modes of the membrane to obtain a vibratory response amplified over a larger frequency bandwidth, which constitutes a drawback.